Multiple-piece golf ball

ABSTRACT

A multiple-piece golf ball includes a core positioned at the center of the golf ball; an intermediate layer surrounding the core; a cover further surrounding the intermediate layer; at least one high specific gravity member having a higher specific gravity than the core, the intermediate layer and the cover. The at least one high specific gravity member is arranged such that a moment of inertia of the golf ball with respect to an x-axis Ix and a moment of inertia of the golf ball with respect to a y-axis Iy are substantially the same, a moment of inertia of the golf ball with respect to a z-axis Iz is greater than Ix and Iy, and the difference between Iz and Ix or Iy is at least approximately 3. The x-axis, y-axis, and z-axis intersect orthogonally at the center of the golf ball.

BACKGROUND OF THE INVENTION

The present invention relates to a multiple-piece golf ball.

While there are many factors that affect the distance of a golf ball, three factors, the initial velocity of the ball, the launch angle, and the spin rate are extremely important. Of these, the ball spin is an essential element in lifting the golf ball and extending its distance. However, if the spin rate is excessive, the ball can fly upward or the ball line can be short after falling, so that the distance of the ball is not extended. The spin rate is greatest at the time the ball is struck, and gradually decreases, that is, attenuates, as the ball flies.

Japanese Patent Application Publication No. 2002-515808 discloses that, with the object of providing a golf ball that generates a lower spin rate for a golfer with a low level of skill, an outer layer of a golf ball includes a discrete region of a weighting material, the weighting material being visible at the exterior surface of the golf ball so as to increase the moment of inertia of the golf ball.

Also, Japanese Patent Application Publication No. 2002-536136 discloses that, with the object of providing a golf ball that generates a lower spin rate for a golfer with a low level of skill, a golf ball includes a soft core having a Riehle compression of at least 75 and a hard cover having a Shore D hardness of at least about 65 disposed around the core, the cover including at least one region of a weighting material, and at least one other region of material less dense than the weighting material so as to increasing the moment of inertia of the golf ball.

SUMMARY OF THE INVENTION

These publications, however, disclose a golf ball having an increased moment of inertia to reduce the spin rate thereof, but fail to disclose the relationship between an initial spin rate and a decay rate thereof during the time the golf ball is in flight. In a case in which the initial spin rate is high, increasing the spin decay rate during flight can arrive at a proper spin rate. However, in a case in which the initial spin rate is low, generating a high spin decay rate during flight cannot increase the travel distance of the golf ball.

Accordingly, it is an object of the present invention to provide a multiple-piece golf ball that generates a high spin decay rate in a case in which an initial spin rate at the time that the golf ball is struck is high and generates a low spin decay rate in a case in which the initial spin rate is low, thereby increasing the travel distance of the golf ball even in both cases in which the initial spin rate is either high or low.

In order to achieve the above-noted object, the present invention is a multiple-piece golf ball including a core disposed at the center of the golf ball, an intermediate layer surrounding the core, a cover further surrounding the intermediate layer, and at least one high specific gravity member having a specific gravity that is greater than the core, the intermediate layer and the cover, wherein the at least one high specific gravity member is arranged such that a moment of inertia of the golf ball with respect to an x-axis Ix and a moment of inertia of the golf ball with respect to a y-axis Iy are substantially the same, a moment of inertia of the golf ball with respect to a z-axis Iz is greater than Ix and Iy, and the difference between Iz and Ix or Iy is at least approximately 3, the x-axis, y-axis, and z-axis intersecting orthogonally at the center of the golf ball.

There may be one high specific gravity member or a plurality thereof. The specific gravity of the high specific gravity member is preferably at least about 2. The high specific gravity member preferably includes a metal or a metal compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top cross-sectional view showing one embodiment of a multiple-piece golf ball according to the present invention.

FIG. 2 is a side cross-sectional view of the multiple-piece golf ball shown in FIG. 1.

FIG. 3 is elevation view of the intermediate layer of the multiple-piece golf ball shown in FIG. 1.

FIG. 4 is a top cross-sectional view showing another embodiment of a multiple-piece golf ball according to the present invention.

FIG. 5 is a side cross-sectional view showing the multiple-piece golf ball shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although embodiments of a multiple-piece golf ball according to the present invention are described below, with reference made to the accompanying drawings, the present invention is not limited to these embodiments. The accompanying drawings were made giving priority to understanding of the present invention, and they are not drawn to scale.

As shown in FIGS. 1 and 2, a multiple-piece golf ball 1 according to this embodiment mainly includes a core 10 disposed at the center of the ball, an intermediate layer 20 surrounding the exterior of the core, and a cover 30 that surrounds the exterior of the intermediate layer. Three axes intersecting orthogonally at the center of the golf ball 1 are taken as the x-axis, the y-axis and the z-axis. A high specific gravity member 40 having an annular shape which has the same center as the center of the ball is arranged on a plane including two axes of the three axes, e.g., on the x-y plane as shown in FIGS. 1 and 2.

The golf ball of the present invention is configured so that, of the moments of inertia Ix, Iy, and Iz of the golf ball with respect to each of the three axes that intersect orthogonally at the center of the golf ball, the two moments inertia Ix and Iy are substantially the same, and the remaining one moment of inertia Iz is greater than the two moments of inertia Ix and Iy, the difference being at least approximately 3. By having this configuration, in a case in which the golf ball 1 is struck so that the x-axis or the y-axis is the axis of backspin rotation, because moments of inertia Ix and Iy are small, the spin decay rate during flight is high. Thus, in a case, for example, in which the initial spin rate is high, the spin rate during flight is reduced to within a proper range, enabling the distance to be extended. In contrast, in a case in which the golf ball 1 is struck so that the z-axis is the axis of backspin rotation, because moment of inertia Iz is large, the spin decay rate during flight is decreased. As a result, for example, in a case in which the initial spin rate is low, the spin rate during flight is maintained to within a proper range, enabling the distance to be extended.

The difference between the moment of inertia Iz and the moment of inertia Ix or Iy is preferably at least about 4, more preferably at least about 5, and still more preferably at least about 6. This is because if the difference of the moment of inertia Iz and the moment of inertia Ix or Iy is too large, there can be non-uniformity in the initial performance, such as a variation in the initial spin, depending on the orientation of the golf ball when it is struck, the difference is preferably less than approximately 30, more preferably less than about 24, still more preferably less than about 23, and still more preferably less than about 22.

The moment of inertia Iz is preferably not greater than about 120, more preferably not greater than about 118, and still more preferably not greater than about 116. The moments of inertia Ix and Iy are preferably at least about 50, more preferably at least about 52, and still more preferably at least about 54.

The high specific gravity member 40 having a specific gravity that is higher than the core 10, the intermediate layer 20, and the cover 30 is arranged in the golf ball 1 so that the moments of inertia Ix, Iy, and Iz with respect to the three axes have the relationship noted above. The core 10, the intermediate layer 20, the cover 30, and the high specific gravity member 40 are described in detail below.

The core 10, although shown in FIG. 1 as being solid, is not limited to being solid, and may be hollow, or may be a foam core formed by insertion of a foaming agent. For example, the specific gravity of the core 10 is preferably less than about 14, and more preferably less than about 13.5, but is not limited thereto. Although there is no limitation on the lower limit of the specific gravity of the core 10, the specific gravity is preferably at least approximately 0.3, and more preferable at least approximately 0.4. As the material for forming the core 10, it is possible to use, for example, a rubber composition formed by a base rubber as the main component, which optionally includes a cross-linking agent, an initiator, a filler, a foaming agent, an antioxidant, and an organosulfur compound. A thermoplastic elastomer, ionomer resin, or mixture thereof may be used as the main component in place of the base rubber.

A rubber (a thermosetting elastomer) may be generally used as the base rubber, and although, for example, a polybutadiene rubber (BR), a styrene-butadiene rubber (SBR), a natural rubber (NR), a polyisoprene rubber (IR), a polyurethane rubber (PU), a butyl rubber (IIR), a vinyl polybutadiene rubber (VBR), an ethylene propylene rubber (EPDM), a nitrile rubber (NBR), and a silicone rubber may be used, there is no limitation to these components. As the polybutadiene rubber (BR), for example, 1,2-polybutadiene or 1,4-cis-polybutadiene or the like may be used.

The Moony viscosity (ML₁₊₄ (100° C.)) of the polybutadiene is desirably at least about 30, preferably at least about 35, more preferably at least about 40, still more preferably at least about 50, and most preferably at least about 52. The upper limit is desirably not greater than about 100, preferably not greater than about 80, more preferably not greater than about 70, and still more preferably not greater than about 60.

The Moony viscosities in the present invention indicate the industrial viscosity index (Japan Industrial Standard JIS K 6300) that is measured by a Moony viscosity gage, which is a type of rotary plastometer, and ML₁₊₄ (100° C.) is used as a unit symbol. Symbol M indicates the Moony viscosity, L indicates the large rotor (L type), 1+4 indicates the pre-heating time of one minute and the rotor rotating time of four minutes, and it indicates that the measurement is performed under the condition of 100° C.

A molecular weight distribution of the polybutadiene Mw/Mn (Mw: A weight average molecular weight, Mn: A number-average molecular weight) is desirably at least about 2.0, preferably at least about 2.2, more preferably at least about 2.4, and still further more preferably at least about 2.6. The upper limit is desirably not greater than about 6.0, preferably not greater than about 5.0, more preferably not greater than about 4.0, and still more preferably not greater than about 3.4. If Mw/Mn is too small, poor workability may result, and if it is too large, poor repulsion may result.

Although the polybutadiene may be synthesized with a Ni or Co catalyst or with a rare-earth element catalyst, one synthesized with a rare-earth element catalyst is particularly preferable, and a widely known rare-earth element catalyst can be used. Exemplary catalysts include those made of a combination of a lanthanide series rare-earth element compound, an organoaluminum compound, an alumoxane, a halogen-bearing compound and, if necessary, a Lewis base.

In the present invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth element compound is particularly advantageous because it enables a polybutadiene rubber having a high 1,4-cis-bond content and a low 1,2-vinyl bond content to be obtained with excellent polymerization activity. Specific examples of these rare-earth element catalysts include those noted in Japanese Patent Application Publication No. 11-35633, which is incorporated by reference.

In the case of the butadiene polymerization in the presence of a rare-earth element catalyst, a solvent may be used or bulk polymerization or vapor phase polymerization may be preferable, without using the solvent, and the polymerization temperature can be generally from about −30° C. to about 150° C., and preferably from about 10° C. to about 100° C.

The above-noted butadiene may be obtained by following polymerization by the above-noted rare-earth element catalyst by reacting a terminal modifier with the activation terminal of the polymer. The specific embodiment of the terminal modifier and the method of activation include, for example, those noted, and methods in the Japanese Patent Application Publications 11-35633, 7-268132, and 2002-293996, which are incorporated by reference.

The butadiene is preferably to be blended at least about 60 wt % into the base rubber, more preferably at least about 70 wt %, still more preferably at least about 80 wt %, and most preferably at least 90 wt %. The upper limit of the mixture ratio of the butadiene is preferably about 100 wt %, more preferably about 98 wt %, and still more preferably about 95 wt %. By mixing the polybutadiene within this range, a golf ball having good resilience can be obtained.

Rubbers other than the above-described polybutadiene may be blended in combination with the polybutadiene, as long as the object of the present invention is not lost. Particularly preferable rubbers for the combination include a styrene-butadiene rubber, a natural rubber, a polyisoprene rubber, and an ethylene-propylene-diene rubber. These may be used alone or in combination of two or more thereof.

Although there is no particular limitation with regard to the co-crosslinking agent, it is preferable that, for example, an α,β-unsaturated carboxylic acid or a metal salt thereof be used. As an α,β-unsaturated carboxylic acid or the metal salt, there are, for example, an acrylic acid, methacrylate, and a zinc salt, a magnesium salt, and a calcium salt thereof. Although there is no particular limitation with regard to the co-crosslinking agent content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 5 parts by weight, and more preferably at least about 10 parts by weight. The co-crosslinking agent content is also preferably not greater than about 70 parts by weight, and more preferably not greater than about 50.

Although no particular limitation is imposed on the initiator, the use of an organic peroxide is preferable. Although no particular limitation is imposed on the initiator content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 0.10 parts by weight, more preferably at least about 0.15 parts by weight, and still more preferably at least about 0.30 parts by weight, but preferably not greater than 8 parts by weight, and more preferably not greater than 6 parts by weight.

In order to increase the specific gravity of the core 10, a metal or metallic compound with a specific gravity of at least about 2 is desirable as a filler. Although examples of such a metal or a metallic compound include silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, and manganese oxide, there is no particular limitation thereto. The filler is preferably formed as a powder. Although there is no particular limitation with respect to the filler content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 5 parts by weight, more preferably at least about 10 parts by weight, still more preferably at least about 15 parts by weight, and most preferably at least 25 parts by weight. The upper limit of the filler content is preferably about 1000 parts by weight, more preferably about 980 parts by weight, and still more preferably about 960 parts by weight.

Although no particular limitation is imposed on the foaming agent, for example, an azodicarbonamide, azobisisobutyronitrile, dinitrosopentamethylenetetramine, p-toluenesulfonyl hydrazide, p,p′-oxybis(benzenesulfonylhydrazide), and sodium hydrogencarbonate may be used. Although no limitation is imposed on the foaming agent content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 5 parts by weight, and more preferably at least about 10 parts by weight. The foaming agent content is preferably not greater than about 30 parts by weight, and more preferably not greater than about 25 parts by weight.

The core 10 shape is substantially spherical. The outside diameter of the core 10 is preferably not greater than about 40 mm, more preferably not greater than about 39 mm, and still more preferably not greater than about 36 mm. A lower limit to the outer diameter of the core 10 is preferably at least about 4 mm, more preferably at least about 5 mm, still more preferably at least about 6 mm, still more preferably at least about 7 mm, and most preferably at least about 8 mm.

The core 10 is not limited to a single layer configuration, as shown in the FIG. 1, but may include plural layers. For example, the core may be formed by two layers having a center core located at the centermost of the golf ball and a core layer that covers the center core. In the case in which the core is made of a multilayer structure, the thickness of the core layer is preferably at least about 0.8 mm, more preferably at least about 1.0 mm, and still more preferably at least about 1.4 mm. The upper limit of the thickness of the core layer is desirably about 10 mm, preferably about 8 mm, more preferably about 6 mm, and still more preferably about 3 mm. The above-noted core materials can be used as the material for both the center core and the core layer.

Although there is no limitation in this regard, the specific gravity of the intermediate layer 20 is preferably at least about 0.2, more preferably at least about 0.3, and still more preferably at least about 0.4. The specific gravity of the intermediate layer 20 is preferably no greater than about 1.2, more preferably no greater than about 1.1, and still more preferably no greater than about 1.08. Although there is no limitation in this regard, the material of the intermediate layer 20 may be selected from a thermosetting elastomer, a thermoplastic elastomer, an ionnomer resin, or a combination thereof.

A polybutadiene rubber (BR), a styrene-butadiene rubber (SBR), a natural rubber (NR), a polyisoprene rubber (IR), a polyurethane rubber (PU), a butyl rubber (IIR), a vinyl polybutadiene rubber (VBR), an ethylene propylene rubber (EPDM), a nitrile rubber (NBR), and a silicone rubber may be used as the thermoset elastomer, but there is no limitation to these components.

The Moony viscosity (ML₁₊₄ (100° C.)) of the polybutadiene is desirably at least about 30, preferably at least about 35, more preferably at least about 40, still more preferably at least about 50, and most preferably at least about 52. The upper limit is desirably not greater than about 100, preferably not greater than about 80, more preferably not greater than about 70, and still more preferably not greater than about 60.

The Moony viscosities in the present invention indicate the industrial viscosity index (Japan Industrial Standard JIS K 6300) that is measured by a Moony viscosity gage, which is a type of rotary plastometer, and ML₁₊₄ (100° C.) is used as a unit symbol. Symbol M indicates the Moony viscosity, L indicates the large rotor (L type), 1+4 indicates the pre-heating time of one minute and the rotor rotating time of four minutes, and it indicates that the measurement is performed under the condition of 100° C.

A molecular weight distribution of the polybutadiene Mw/Mn (Mw: A weight average molecular weight, Mn: A number-average molecular weight) is desirably at least about 2.0, preferably at least about 2.2, more preferably at least about 2.4, and still further more preferably at least about 2.6. The upper limit is desirably not greater than about 6.0, preferably not greater than about 5.0, more preferably not greater than about 4.0, and still more preferably not greater than about 3.4. If Mw/Mn is too small, poor workability may result, and if it is too large, poor repulsion may result.

Although the polybutadiene may be synthesized with a Ni or Co catalyst or with a rare-earth element catalyst, one synthesized with a rare-earth element catalyst is particularly preferable, and a widely known rare-earth element catalyst can be used. Exemplary catalysts include those made up of a combination of a lanthanide series rare-earth element compound, an organoaluminum compound, an alumoxane, a halogen-bearing compound and, if necessary, a Lewis base.

In the present invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth element compound is particularly advantageous because it enables a polybutadiene rubber having a high 1,4-cis-bond content and a low 1,2-vinyl bond content to be obtained with excellent polymerization activity. Specific examples of these rare-earth element catalysts include those noted in Japanese Patent Application Publication No. 11-35633, which is incorporated by reference.

In the case of the butadiene polymerization in the presence of a rare-earth element catalyst, a solvent may be used or bulk polymerization or vapor phase polymerization may be preferable, without using the solvent, and the polymerization temperature can be generally from about −30° C. to about 150° C., and preferably from about 10° C. to about 100° C.

The above-noted butadiene may be obtained by following polymerization by the above-noted rare-earth element catalyst by reacting a terminal modifier with the activation terminal of the polymer. The specific embodiment of the terminal modifier and the method of activation include, for example, those noted and methods in the Japanese Patent Application Publications 11-35633, 7-268132, and 2002-293996, which are incorporated by reference.

The butadiene is preferably to be blended at least about 60 wt % into the base rubber, more preferably at least about 70 wt %, still more preferably at least about 80 wt %, and most preferably at least 90 wt %. The upper limit of the mixture ratio of the butadiene is preferably about 100 wt %, more preferably about 98 wt %, and still more preferably about 95 wt %. By mixing the polybutadiene within this range, a golf ball having good resilience can be obtained.

Rubbers other than the above-described polybutadiene may be blended in combination with the polybutadiene, as long as the object of the present invention is not lost. Particularly preferable rubbers for the combination include a styrene-butadiene rubber, a natural rubber, a polyisoprene rubber, and an ethylene-propylene-diene rubber. These may be used alone or in combination of two or more thereof.

Although there is no limitation in this regard, as the thermoplastic elastomer, for example, it is possible to use a polyester thermoplastic elastomer, a polyurethane thermoplastic elastomer, a polyamide thermoplastic elastomer, and a polyolefin thermoplastic elastomer.

As the ionomer resin, although there is no limitation in this regard, the following components (a) and/or (b) are to be the base resins. Optionally, the component (c) may be added to the base resin. The component (a) includes at least one of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or the metal salts thereof, the component (b) includes an olefin-unsaturated carboxylic acid random binary copolymer and/or metal salts thereof, and the component (c) includes crystalline polyolefin blocks, and a thermoplastic block copolymer having a polyethylene-butylene random copolymer.

The weight-average molecular mass (Mw) of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or metal salts thereof forming the component (a) is preferably at least about 100,000, more preferably at least about 110,000, and still more preferably at least about 120,000. The upper limit is preferably about 200,000, more preferably about 190,000, and still more preferably about 170,000. The ratio of the weight-average molecular mass (Mw) and the number-average molecular mass (Mn) of the above-noted copolymer is preferably about 3.0 to about 7.0.

The component (a) is a copolymer including an olefin, for example, in which the number of carbons is at least 2 as the olefin in the component (a), but the upper limit is preferably 8, and particularly preferably 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene, octene and the like. Ethylene is particularly preferred.

Examples of unsaturated carboxylic acids in the component (a) include acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Acrylic acid and methacrylic acid are particularly preferred.

The unsaturated carboxylic acid ester in the component (a) includes, for example, a lower alkyl ester of the above-described unsaturated carboxylic acid. Specific examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) is particularly preferred.

The random copolymer in the component (a) can be obtained by random copolymerizing the above-noted materials by a known method. The content of unsaturated carboxylic acid (acid contents) present in the random copolymer is usually at least about 2 wt %, preferably at least about 6 wt %, and more preferably at least about 8 wt %. The upper limit may be about 25 wt %, is preferably about 20 wt %, and more preferably about 15 wt %. If the acid content is low, the resilience may decrease, whereas if it is high, the workability of the material may decrease.

The metal ion of the copolymer in the component (a) can be obtained by partially neutralizing the acid groups on the random copolymers of the component (a) with metal ions.

Illustrative examples of metal ions for neutralizing the acid groups include ions such as Na, K, Li, Zn, Cu, Mg, Ca, Co, Ni, and Pb, and are preferably ions, such as, Na, Li, Zn, Mg, and Ca, and Zn ion is more preferably recommended. Although the degree of the neutralization of the random copolymer in these metal ions is not particularly limited, it is normally at least about 5 mol %, preferably at least about 10 mol %, particularly at least about 20 mol %. The upper limit is usually about 95 mol %, preferably 90 mol %, and particularly about 80 mol %. If the degree of neutralization exceeds about 95 mol %, formability may decrease. In a case in which the upper limit is about 5 mol %, because it is necessary to increase the additive amount of inorganic metal compounds to the component (e), this might be disadvantageous in view of cost. Such neutralization material can be obtained using a known method, for example, a compound such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide of the above-noted metal ions being introduced with respect to the above-noted random copolymer.

Illustrative examples of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer which constitutes the component (a) include particularly, in terms of commercial names, “Nucrel AN 4318”, “Nucrel AN4319”, and “Nucrel AN4311” (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the metal salts of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer include specifically, in terms of commercial names, “Himilan AM7316”, “Himilan AM7331”, “Himilan 1855” and “Himilan 1856” (all products of DuPont-Mitsui Polychemicals Co., Ltd.) and, in terms of commercial names, “Surlyn 6320” and “Surlyn 8120” (all products of DuPont US).

The weight average molecular mass (Mw) of one or more of an olefin-unsaturated carboxylic acid random binary copolymer and the metal salts thereof, which constitutes the component (b), is preferably at least about 100,000, more preferably at least about 110,000 and still more preferably at least about 120,000. The upper limit is preferably about 200,000, more preferably about 190,000, and still more preferably not more than about 170,000. The ratio of the weight-average molecular mass (Mw) and the number-average molecular mass (Mn) of the above-noted copolymer is preferably about 3.0 to about 7.0.

The ratio in which the copolymer of the component (e) occupies the overall base resin is about 0 wt % to about 20 wt %. The lower limit is preferably about 1 wt %. The upper limit is preferably about 17 wt %, more preferably about 10 wt %, more preferably about 8 wt %, and still more preferably about 5 wt %.

Illustrative examples of the olefin-unsaturated carboxylic acid random binary copolymer and the metal salts thereof, which constitute the component (b), include specifically, in terms of commercial names, “Nucrel 1560”, “Nucrel 1525”, and “Nucrel 1035” (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the metal salts of the olefin-unsaturated carboxylic acid random binary copolymer include specifically, in terms of commercial names, “Himilan 1605”, “Himilan 1601”, “Himilan 1557”, “Himilan 1705” and “Himilan 1706” (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and in terms of commercial names, “Surlyn 7930” and “Surlyn 7920” (all products of DuPont US).

Illustrative examples of the thermoplastic block copolymer having crystalline polyolefin blocks, or polyethylene-butylene random copolymers as the component (c) include crystalline polyethylene blocks (E) as the hard segment, and blocks which constitute the relatively random copolymer (EB) of the ethylene and the butylene as the soft segment. A block copolymer having an E-EB type structure, or an E-EB-E type structure or the like, in which the hard segment exists at one end or both ends as a molecular structure, is preferably used.

The components (c), that is, the thermoplastic block copolymer having crystalline olefin blocks or polyethylene/butylene random copolymer can be obtained, for example, by hydrogenating the polybutadiene. The polybutadiene used for hydrogenating particularly has in a blocking manner a 1,4-polymerization part in which the 1,4-bond is about 95 to 100 wt %, and the polybutadiene in which the 1,4-bond content in the overall butadiene structures is about 50 wt % to 100 wt %, and more preferably about 80 wt % to 100 wt % is appropriately used as a bonding mode in the butadiene structure. That is, the polybutadiene in which the 1,4-bond content is about 50 wt % to 100 wt %, and more preferably about 80 wt % to 100 wt % and the polybutadiene having in a blocking manner about 95 to 100 wt % of 1,4-bond are preferably used.

The above-noted E-EB-E type thermoplastic block copolymer is preferably obtained by hydrogenating the polybutadiene in which the ends of the molecular chain is 1,4-bond-rich 1,4-polymer and the 1,4-bond and 1,2-bond are mixed in the intermediate part. In this case, the amount of hydrogen additive (the conversion rate to the saturated bond of the double bond in the polybutadiene) in the hydrogen additive is preferably about 60% to about 100%, and more preferably about 90% to 100%. If the amount of the hydrogen additive is too small, deterioration such as gelatinization may be caused in the blending process with ionomer resin and the like. Also, there might be a problem of an impact resistance as an intermediate layer, caused when forming the golf ball.

In the block copolymer having an E-EB type, an E-EB-E type structure or the like in which the hard segment exists at one end or both ends as a molecular structure, which is preferably used as the thermoplastic block copolymer, the amount of hard segment is preferably about 10 wt % to about 50 wt %. If the amount of the hard segment is excessive, the object of the present invention may not be achieved effectively due to the lack of flexibility, and if the amount of the hard segment is too small, a problem of formability of the blend may be caused.

The melt index of the above thermoplastic block copolymer at 230° C., and a test load of 21.2 N is preferably about 0.01 g/10 min to about 15 g/10 min, and more preferably about 0.03 g/10 min to about 10 g/10 min. In the case of exceeding the above-noted range, problems at the time of the injection molding, such as a weld, a sink, a short or the like may occur. The surface hardness of the thermoplastic block copolymer is preferably 10 to 50. If the surface hardness is too low, the durability with respect to repeated impact of the golf ball may decrease. On the other hand, if the surface hardness is too high, the resilience of the blend with the ionomer resin may decrease. The number-average molecular weight of the thermoplastic block copolymer is preferably from about 30,000 to about 800,000.

A commercially available product may be used as the thermoplastic block copolymer having the above crystalline polyolefin blocks, or a polyethylene/butylene random copolymer, and illustrative examples include Dynaron 6100P, 6200P and 6201B of JSR Corporation. In particular, Dynaron 6100P is a block polymer having crystalline olefin blocks at the ends, and may be used preferably in the present invention. These olefin series thermoplastic elastomers may be used as one type alone or may be used by blending with two or more types.

The material of the intermediate layer 20, as the component (d) with respect to 100 parts by weight of the above-noted resin components (a) to (c), can be mixed at about 5 to 100 parts by weight of a fatty acid or derivative thereof having a molecular weight from about 280 to about 1500, and as the component (e), can be mixed with from about 0.1 parts by weight to about 10 parts by weight of the basic inorganic metal compounds, which can neutralize acid groups in the above-noted components (a), (b) and (d).

Component (d) is a fatty acid or a fatty acid derivative having a molecular weight of at least 280, but no greater than 1500. Compared with the above-noted components (a) to (c), this component has a very low molecular weight, and by contributing a prominent increase in the melt viscosity of the mixture, helps in improving the flow properties of heated mixture. The fatty acid (or derivatives thereof) of the component (d) includes a relatively high content of acid groups (or derivatives thereof), and its addition suppresses an excessive loss in resilience.

The fatty acid or fatty acid derivative of component (d) may be an unsaturated fatty acid (or derivative thereof) containing a double bond or triple bond on the alkyl group, or it may be a saturated fatty acid (or derivative thereof) in which the bonds on the alkyl group are only single bonds. It is recommended that the number of carbons in one molecule be usually at least 18, the upper limit being 80, and in particular, 40. Fewer carbons may deteriorate the heat resistance and may also make the acid group content so large as not to prevent the attainment of the desired flowability due to interactions with acid groups present in the base resin. On the other hand, if there are many carbons, the molecular weight increases, so that the flowability may decrease and make use as a material difficult.

As the fatty acid of component (d), specifically, use of stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid can be exemplified; in particular, stearic acid, arachidic acid, behenic acid and lignoceric acid are preferred.

One substituted with a proton included in the acid group of the fatty acid may be cited as the fatty acid derivative of component (d), and metallic soaps replaced with a metal ion may be cited as an example thereof. As examples of the metal ion for use in metallic soaps, ions such as Li, Ca, Mg, Zn, Mn, Al, Ni, Fe, Cu, Sn, Pb, and Co may be cited. Of these ions, Ca, Mg, and Zn are particularly preferred. The Fe ion may be bivalent or trivalent.

Specific examples of fatty acid derivatives that may be used as component (d) include a magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. In particular, the magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate may be preferably used.

It is recommended that component (d) be included in an amount, per 100 parts by weight of the above-noted base resin, of usually at least about 5 parts by weight, preferably at least about 8 parts by weight, more preferably at least about 20 parts by weight, and still more preferably at least 40 parts by weight, and an upper limit, usually about 100 parts by weight, preferably about 90 parts by weight, more preferably about 80 parts by weight, and still more preferably about 70 parts by weight.

In the case of using one or more of the above-noted components (a) and (d), a widely known metal soap-modified ionomer may be used (U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and PCT International Publication No. 98/46671 being incorporated by reference).

Component (e) may be added as a basic inorganic metal compound capable of neutralizing acid groups in above-noted components (a), (b) and (c). As described in the conventional example, when components (a), (b) and (d) alone, particularly, only a metal modified ionomer resin (e.g., the metal soap-modified ionomer resins alone cited in the above-noted patent publications) are used, as shown below, the metallic soap and unneutralized acid groups present on the ionomer undergo exchange reactions during mixture under heating, thereby generating a fatty acid. Because the thus-generated fatty acid has a low thermal stability and readily vaporizes during molding, it may cause molding defects, and the fatty acid thus-generated is deposited on the surface of the molded material, and this may cause a substantially lower paint film adhesion. Component (e) will be mixed in to solve these problems.

The heated mixture used in the present invention is a blend, as described above, including compound (e) as an essential ingredient, the basic inorganic metal compound neutralizing acid groups included in the above-noted components (a), (b) and (d). By blending component (e), the acid groups in the above components (a), (b) and (d) are neutralized and, through synergistic effects from the blending of each of these components, this contributes as well to an increase in the thermal stability of the heated mixture and gives it good moldability, and also enhances the resilience as a golf ball.

The component (e) is a basic inorganic metal compound capable of neutralizing acid groups within the above-noted components (a), (b) and (d), and is preferably recommended as a monoxide or hydroxide, which has a high reactivity with the ionomer resin and contains no organic acids in the reaction by-products, thus enabling the degree of neutralization of the heated mixture to be increased without a loss of thermal stability.

As the metal ions used in the basic inorganic metal compounds, ions, for example, Li, Na, K, Ca, Mg, Zn, Al, Ni, Fe, Cu, Mn, Sn, Pb, and Co can be cited, and as an inorganic metal compound there are basic inorganic fillers including these metallic ions, specifically, a magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate can be cited. A monoxide or a hydroxide is recommended as noted above. The magnesium oxide and calcium hydroxide, which have a high reactivity with the ionomer resin, may be preferably used.

The component (e) is usually about 0.1 to about 10 parts by weight, per 100 parts by weight of the above-noted base resin. The lower limit is preferably about 0.5 parts by weight, more preferably about 1 part by weight. The upper limit is preferably about 5 parts by weight, more preferably about 3 parts by weight.

The heated mixture to be used in the present invention is mixed with the above-described components (a) to (e), thus improving thermal stability, moldability and resilience, and it is recommended that at least about 70 mol %, preferably at least about 80 mol %, and more preferably at least about 90 mol %, of the acid groups within every heated mixture to be used in the present invention is neutralized. Such a high degree of neutralization makes it possible to more reliably suppress the exchange reactions that cause a problem when only above-noted components (a), (b) and a fatty acid (derivative) are used as described above, thus preventing the generation of fatty acid and dramatically increasing thermal stability, enabling products of good formability and very excellent resilience compared to prior-art ionomer resins.

In this case, for the neutralization of the heated mixture of the present invention to achieve more reliably both a high degree of neutralization and flowability, it is recommended that the acid groups in the above-noted heated mixture be neutralized with transition metal ions and with at least one of alkali metal and alkaline earth metal ions. Because neutralization with transition metal ions results in a weaker ionic cohesion than with alkali (earth) metal ions, a part of the acid groups within the heated mixture is neutralized to be able to achieve a prominent improvement in flowability.

Various optional additives may be added, if necessary, in the above-noted heated mixture in the present invention, for example, fillers, pigments, dispersants, antioxidants, ultraviolet absorbers, and light stabilizers. In order to improve the feel of the ball upon impact, in addition to the above-noted essential components, various non-ionomeric thermoplastic elastomers may be blended, for example, styrene thermoplastic elastomers, ester thermoplastic elastomers, and urethane thermoplastic elastomers, and the use of styrene thermoplastic elastomers is particularly preferred.

A method of adjusting the heated mixture, for example, is mixing during heating using in internal mixer, such as a two-screw extruder, Banbury mixer, kneader, and the like, under the conditions for heating and mixing at the temperature of, for example, about 150° C. to about 250° C. The method of forming the intermediate layer using the above-noted heated mixture is not particular limited to the above, and it may be formed using, for example, injection molding or compression forming, or the like. In the case in which the injection molding method is used, after disposing the prefabricated solid core at the predetermined position of the injection mold, the method for introducing the above-noted material into the mold may be used. In the case in which the compression forming method is used, after forming one pair of half-cups using the above-noted material, the cups surround the core directly or via the intermediate layer, thereby enabling pressurizing and heating thereof within the mold. When applying pressure and heat for forming the ball, the conditions of the temperature at about 120° C. to about 170° C. and the time period of about 1 to 5 minutes may be used.

The intermediate layer 20 may optionally include a filler or a foaming agent, in addition to the main component such as the rubber, thermoplastic elastomer, and ionomer resin as described above. Examples of the filler include barium sulfate, zinc oxide, manganese oxide, and tungsten, but there is no particular limitation thereto. The filler is preferably formed as a powder. Although there is no particular limitation with respect to the filler content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 5 parts by weight, more preferably at least about 20 part by weight, and still more preferably at least about 25 parts by weight. The upper limit of the filler content is preferably about 1000 parts by weight, more preferably about 980 parts by weight, and still more preferably about 960 parts by weight.

Although no particular limitation is imposed on the foaming agent, for example, an azodicarbonamide, azobisisobutyronitrile, dinitrosopentamethylenetetramine, p-toluenesulfonyl hydrazide, p,p′-oxybis(benzenesulfonylhydrazide), and sodium hydrogencarbonate may be used. Although no limitation is imposed on the foaming agent content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 2 parts by weight, and more preferably at least about 3 parts by weight. The foaming agent content is preferably not greater than about 30 parts by weight, and more preferably not greater than about 25 parts by weight.

The thickness of the intermediate layer 20 is not limites but may be, for example, preferably at least about 1 mm, more preferably at least about 2 mm, and still more preferably at least about 3 mm. The upper limit of the thickness of the intermediate layer 20 is preferably about 15 mm, more preferably about 13 mm, and still more preferably about 11 mm.

Although the intermediate layer 20 is, as shown in FIG. 1, configured of a single layer, it is not limited to this configuration, and may be configured of plural layers such as two or more layers.

Although the specific gravity of the cover 30 is not limited, it is preferably at least about 0.91, and more preferably at least about 0.93, but preferably not greater than about 1.3, and more preferably not greater than about 1.2. An ionomer resin, a polyurethane thermoplastic elastomer, a thermoset polyurethane, or mixture thereof may be used as the material for the cover 30, but there is no limitation thereto.

Although the thickness of the cover 30 is not limited, it is preferably at least about 0.2 mm, and more preferably at least about 0.4 mm, but preferably not greater than about 4 mm, more preferably not greater than about 3 mm, and still more preferably not greater than about 2 mm.

The high specific gravity member 40 has a higher specific gravity than either of the core, the intermediate layer 20 and the cover 30. The specific gravity of the high specific gravity member 40, although it is not limited to these values, from the standpoint of degree of freedom, is preferably at least about 2, more preferably at least about 2.5, and still more preferably at least about 4. On the other hand, the specific gravity of the high specific gravity member 40 is preferably no greater than about 22, and more preferably no greater than about 20, because if the specific gravity of the high specific gravity member 40 is too high, it is difficult in the manufacturing process to achieve a prescribed moment of inertia by only a small non-uniform arrangement of the high specific gravity member. It is preferable to use a metal composition as a high specific gravity member 40 having this specific gravity.

The metal composition may include a metal or a metallic compound. Although exemplary metal or metallic compounds include silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, and manganese oxide, there is no limitation imposed thereon.

The metal composition may also include binder for forming the powder of the above-noted metals or the metal compound into a prescribed shape. As the binder, it is preferable to use rubber or a thermosetting resin, and it is possible to use the same material as the above-noted base rubber. The amount mixture of the metal or metal compound powder in the case of using a binder is, with respect to 100 parts by weight of base rubber, preferably at least 200 parts by weight, more preferably at least about 250 parts by weight, still more preferably at least about 350 parts by weight, and most preferably at least about 400 parts by weight. The amount of mixture of the metal or the metal compound is preferably no more than about 2000 parts by weight, more preferably no more than about 1900 parts by weight, still more preferably no more than about 1600 parts by weight, and most preferably no more than about 1500 parts by weight.

Regarding the high specific gravity member 40, if the transmission loss of the material is large in the high frequency range of 1000 Hz to 4000 Hz, because a sound insulation effect can be expected, the transmission loss of the specific gravity-enhancing members 40 is preferably at least about 10 dB in the high frequency range of 1000 Hz to 4000 Hz, more preferably at least about 15 dB, and still more preferably at least about 30 dB. On the other hand, it is preferably not greater than about 60 dB, more preferably not greater than about 55 dB, and still more preferably not greater than about 40 dB.

Although an annular high specific gravity member 40 having a concentricity with the center of the golf ball 1 is disposed on the x-y plane surface in FIGS. 1 and 2, this is not a limitation, as long as the position and the number of the high specific gravity members 40 are such that the moments of inertia on the three axes, Ix, Iy, and Iz satisfy the condition Ix=Iy≦Iz−3. As shown as in FIGS. 4 and 5, for example, on the x-y plane, four high specific gravity members 40 a to 40 d are disposed at the position of the vertices of a regular quadrangle having the center of the golf ball 1 as its center of gravity, thereby enabling satisfaction of the above relationship as well. The number of high specific gravity members 40 is not limited, and there can be a plurality, such as 2, 4, 6, 8, 10, 12, and 20, and the high specific gravity members can be disposed at the vertices of a regular polygon, such as a square, hexagon, octagon, decagon, dodecagon and icosagon. Although the upper limit of the number of the high gravity member 40 is not particularly limited, it is preferably about 600, and more preferably about 500.

The thickness of a sheet-shaped or plate-shaped high specific gravity member 40 is preferably at least about 0.01 mm, more preferably at least about 0.02 mm, still more preferably at least about 0.09 mm, and most preferably at least about 0.10 mm. Because the sagging hardness of the golf ball can become large or the sound of the ball when struck can become unpleasant if the thickness of the high specific gravity member 40 is too great, the thickness of the high specific gravity member 40 is preferably no greater than about 2.0 mm, more preferably no greater than about 1.75 mm, still more preferably no greater than about 0.75 mm, and most preferably no greater than about 0.5 mm.

The shape of the high gravity members 40, although as shown in FIGS. 1 and 2 it is a planar annular sheet-shape or plate-shape, is not limited, as long as the above-described relationship between moments of inertia is satisfied. For example, by providing a plurality of the high specific gravity members 40, it can be spherical, a circular cone, or a polygonal pyramid such as a triangular pyramid, or a quadrangular pyramid, or a polyhedral body such as a regular tetrahedron or regular hexahedron. In the case of a sheet-shaped or plate-shaped high specific gravity member 40, in addition to a planar annular shape, the shape of the high specific gravity member 40 may be, for example, a planar triangular shape or a planar quadrangular shape.

Depending upon the disposition of the high specific gravity members 40, and depending upon the location on the ball that is struck when the golf ball is struck, there might be a variation in the initial performance, such as the spin rate, initial velocity, and launch angle. However, by adjusting the material, shape, and hardness of the high specific gravity member 40, it is possible to reduce the differences in initial performance due to the position at which the ball is struck. By making it possible to achieve uniform initial performance regardless of the point of impact on the ball, it is possible to achieve stable shots such as approaches to the green and bunker shots.

Although the portion of the overall volume of the golf ball 1 occupied by the volume of the high specific gravity member 40 is not particularly limited, as long as the above-described relationship between the moments of inertia is satisfied, it is preferably at least about 0.001%, more preferably at least about 0.005%, still more preferably at least about 0.01%, still more preferably at least about 0.05%, and most preferably at least about 0.1%. The portion of volume is also preferably no greater than about 12.5%, more preferably no greater than about 12%, still more preferably no greater than about 11.5%, and most preferably no greater than about 11%.

In the case in which the high specific gravity members 40 are disposed along the outer peripheral surface of the intermediate layer 20, as shown in FIGS. 1 and 2, a depression part 22 having a dimension the same as the high specific gravity members 40 is formed in the intermediate layer 20 for the purpose of fitting the high specific gravity member 40 thereinto. The method of forming the intermediate layer having the depression part 22 is not limited, and includes such methods as that of placing the core into a mold that is internally spherical having a protrusion on the inside surface and injecting the intermediate layer material and, as shown in FIG. 3, fabricating two half cup-shaped pre-forms of the intermediate layer material in the shape and fitting them together over the core. In the case of disposing the high specific gravity member 40 in the region of the core 10, by using the core material in the above-noted method, it is possible to form a core having a depression part for the fitting thereinto of the high specific gravity member 40 in the same manner.

The intermediate layer 20 may also be formed so that there is a void into which material is not filled. This void can be, for example, a void having the same shape as the concavity 22 shown in FIG. 3, or a hole-type void through which the intermediate layer 20 passes, although it is not limited to these forms. By providing a void in the intermediate layer 20, it is possible, rather than to dispose the high specific gravity member 40, to lighten the intermediate layer 20 and maintain the overall weight of the golf ball 1. Also, in addition to using a foaming agent as the material of the intermediate layer 20, it is possible to provide a void in the intermediate layer 20. The above-described foaming agent may be used as the foaming agent. Although the amount of mixture of foaming agent is not particularly limited, with 100 parts by weight of the main component, it is preferably at least about 2 parts by weight and more preferably at least about 3 parts by weight. It is also preferable that the amount of foaming agent mixed be no greater than about 30 parts by weight and more preferably no greater than about 25 parts by weight.

A plurality of dimples 32 is formed on the surface of the cover 30. The number of dimples 32 on the overall surface of the golf ball 1 is preferably at least about 200, more preferably at least about 250, and still more preferably at least about 300. The upper limit of the number of the dimples 32 is preferably about 500, more preferably about 450, still more preferably about 430, and most preferably about 410. The number of dimples 32 is set within this range, thereby enabling the golf ball 1 to receive a lifting force, and to increase the distance, particularly when using a driver.

The dimple-occupied surface ratio (SR value), which is a ratio of the total area of the dimples which is defined by the plane edges surrounding the borders of the dimples to the spherical surface area of the ball assumed to have no dimples, from the standpoint of fully exhibiting aerodynamic characteristics, is preferably at least about 60%, more preferably at least about 65%, and still more preferably at least about 68%. Although the upper limit of the dimple-occupied surface ratio is not limited, it is preferably about 90%, more preferably about 85%, and still more preferably about 80%.

Also, the spatial volume of each dimple below the flat surface surrounded by the border thereof, when divided by the volume of a cylinder whose base is the said flat surface and whose height is the maximum depth of the dimple from the base is the value V₀, and from the standpoint of achieving a proper trajectory of the ball, V₀ is preferably at least about 0.35. Although the upper limit of the V₀ is not particularly limited, it is preferably about 0.80. Also, the VR value, which is the proportion of the total volume of the dimples, which are formed below the flat surface surrounded by the border of the dimples to the spherical volume of the ball assumed to have no dimples, is preferably at least about 0.6%, more preferably at least about 0.65%, and still more preferably at least about 0.7%. The upper limit of the VR value is preferably 1.0%, and more preferably 0.9%.

Various types of dimples 32 having different sizes of diameters and/or depths may be formed. The number of types of dimples is preferably at least about 3 types, more preferably at least about 4 types, and still more preferably at least about 5 types. The upper limit thereof is preferably about 20 types, more preferably about 15, and still more preferably about 12. By making the number of types of dimples within this range, it is possible to facilitate an increase in the dimple-occupied surface ratio, and to improve the distance.

The shape of the dimple 32 is preferably circular when viewed from above, non-circular when viewed from above, or a combination of these shapes. The average diameter of the dimples is preferably set to at least about 2.8 mm, more preferably at least about 3.5 mm, and still more preferably at least about 3.8 mm. The upper limit thereof may preferably be about 5.0 mm, more preferably about 4.6 mm, and still more preferably about 4.3 mm. The average depth of the dimples 32, from the standpoint of achieving adequate proper trajectory, may be preferably at least about 0.120 mm, more preferably at least about 0.130 mm, and still more preferably at least about 0.140 mm. The upper limit of the average depth may be preferably about 0.185 mm, more preferably about 0.180 mm, and still more preferably about 0.174 mm.

The average diameter means the average of the diameters of all the dimples, and the average depth means the average of the depths of all the dimples. In many cases, the golf ball is painted, and the diameter and the depth of the dimple is measured with the paint coating applied. The measurement of the diameter of the dimples is performed by measuring the width bridged between the points connecting with the land part which is the surface of the golf ball on which the dimples are not formed and the depressed surface. The measurement of the depth of the dimple is performed, when the points connecting the above dimple and the land part are linked to each other on an imaginary plane circle, by measuring vertically the distance between the center thereof to the base surface of the dimple.

As described above, by the present invention having the moments of inertia Ix, Iy, and Iz with respect to the three axes so that the condition Ix=Iy≦Iz−3 is satisfied, it is possible to make the spin decay rate of the golf ball 1 during flight be different between the case in which the x-axis or the y-axis is the backspin rotational axis and the case in which the z-axis is the backspin rotational axis. The spin decay rate in the case in which the golf ball 1 is struck so that the z-axis is the backspin rotational axis is preferably designed to be at least about 0.9%, more preferably at least about 1.0%, and still more preferably at least about 1.1%. The spin decay rate in the case in which the golf ball 1 is struck so that the x-axis or y-axis is the backspin rotational axis is preferably designed to be no greater than about 20%, more preferably no greater than about 15%, and still more preferably no greater than about 7%.

EXAMPLE

Golf balls constituted as shown in Table 1 were made and an experiment was performed to measure the moments of inertia and the distance of the golf balls. The test results are shown in Table 1. The details of mixtures A to E for the materials of the core of Table 1 are shown in Table 2. The details of mixtures F to I (wt %) for the materials of the intermediate layer, the core layer and the cover are shown in Table 3. The details of mixtures J to L (wt %) for the materials of the high specific gravity members are shown in Table 4. For all of Examples 1 to 3 and Comparative Example 1, as shown in FIGS. 1 and 2, the high specific gravity members that were used were annular with an external diameter of 40.7 mm, an internal diameter of 39.7 mm, and a width of 2.5 mm. In order to house such high specific gravity members, depression parts of width 2.5 mm and depth 1 mm were formed in the periphery of the intermediate layer. The depression parts are disposed so as to be shaped with bilateral left-right symmetry with respect to a plane including the z-axis. The bottom surfaces of the depression parts are curved so that the curvature thereof is concentric with the outer peripheral surface of the intermediate layer. The center-facing surface of the high specific gravity member is also curved with the same radius of curvature as the bottom surfaces of the depression parts. The outward-side surface of the high specific gravity member is curved with the same radius of curvature as the outer peripheral surface of the intermediate layer.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Core Outer diameter (mm) 26.0 37.0 36.0 37.6 37.2 Specific gravity 1.15 1.10 1.12 1.13 1.06 Weight (g) 10.6 29.0 27.4 31.2 28.6 Mixture A B E C D Core layer Outer diameter (mm) — — 38.2 — — Specific gravity — — 0.45 — — Weight (g) — — 2.1 — — Mixture — — H — — Intermediate Outer diameter (mm) 40.7 40.7 40.7 40.7 40.7 layer Specific gravity 0.94 0.94 0.94 0.94 1.30 Weight (g) 24.5 8.1 5.6 6.9 10.7 Mixture F F F F G High specific Specific gravity 9.60 4.97 9.60 2.77 — gravity Weight (g) 4.2 2.2 4.2 1.2 — member Mixture or material J K J L — Cover Outer diameter (mm) 42.7 42.7 42.7 42.7 42.7 Specific gravity 16 1.16 1.16 1.16 1.16 Weight (g) 6.5 6.5 6.5 6.5 6.5 Mixture I I I I I Golf ball Weight (g) 45.8 45.8 45.8 45.8 45.8 Moments of X-axis 82 84 81 85 85 inertia Y-axis 82 84 81 85 85 Z-axis 89 87 89 86 85 Distance (m) 2000 rpm, 11 deg, 3.2 2.3 3.2 0.9 0 z-axis 3000 rpm, 11 deg, 2.3 0.3 2.3 0.0 0 x/y-axes

TABLE 2 A B C D E BR730 100 100 100 100 100 Zinc diacrylate 25 25 25 25 25 Perhexa C-40 1.2 1.2 1.2 1.2 1.2 Zinc oxide 23.4 11.7 16.6 5.1 15.0 Zinc stearate 5 5 5 5 5 Organosulfur compound 0.4 0.4 0.4 0.4 0.4 Nocrac NS-6 0.1 0.1 0.1 0.1 0.1

BR730 is the trade name of 1,4-cis-polybutadiene available from JSR Corporation, which was used as the base rubber.

Zinc diacrylate is available from Nippon Shokubai Co., Ltd.

Perhexa C-40 is the trade name of 1,1-bis(tert-butylperoxy) cyclohexane (40% dilution) available from NOF Corporation, which was used as an initiator.

Zinc oxide is available from Sakai Chemical Industry Co., Ltd.

The organosulfur compound is a pentachlorothiophenol zinc salt.

Nocrac NS-6 is the trade name of 2-2′-methylenebis (4-methyl-6-t-butylphenol) available from Ouchi Shinko Chemical Industry Co., Ltd., which was used as an antioxidant.

TABLE 3 F G H I Himilan 1605 66 — 50 — Dynaron 6100P 32 — — — Polytail H 2 — — — Behenic acid 18 — — — Calcium hydroxide 2.3 — — — Himilan 1706 — 35 — Himilan 1557 — 75 15 — Himilan 1855 25 Pandex T8295 — — — 22 Pandex T8290 — — — 65 Magnesium stearate 1.8 Titanium oxide 3.8 Barium sulfate 300 48.1 Foaming agent — 4 — Trimethyrole propane — 1.1 — Hytrel 4001 — — — 13 Polyisocyanate compound — — — 8

Himilan 1605 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Dynaron 6100P is the trade name of a hydrogenated polyolefin thermoplastic elastomer available from JSR Corporation.

Polytail H is the trade name of polyolefin polyol available from Mitsubishi Chemical Corporation.

Behenic acid is available from NOF Corporation.

Calcium hydroxide is available from Shiraishi Kogyo Kaisha Ltd.

Himilan 1557 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Himilan 1855 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Barium sulfate 300 is available from Sakai Chemical Industry Co., Ltd.

Himilan 1706 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

The foaming agent is an ADCA master batch available from Otsuka Chemical Co., Ltd.

Pandex T8295 is the trade name of a polyurethane thermoplastic elastomer available from DIC Bayer Polymer.

Pandex T8290 is the trade name of a polyurethane thermoplastic elastomer available from DIC Bayer Polymer.

Hytrel 4001 is the trade name of a thermoplastic polyether ester elastomer available from DuPont-Toray Co., Ltd.

The polyisocyanate compound is 4,4′-diphenylmethane diisocyanate.

TABLE 4 J K L Tungsten 1900 600 230 BR730 40 40 40 LIR410 60 60 60 Sulfur Z 1 1 1

Tungsten is a tungsten metal (in powder form) available from Nippon Tungsten Co., Ltd. BR730 is the trade name of 1,4-cis-polybutadiene available from JSR Corporation.

LIR410 is the trade name of a liquid isoproylene rubber available from Kuraray Co., Ltd.

Sulfur Z is the trade name of sulfur (in powder form) available from Tsurumi Chemical Industry Co., Ltd.

The moment of inertia of the golf ball was measured using MOI-005, which is an instrument for calculating the moment of inertia and is available from Inertia Dynamics Inc. This measuring instrument measures the moment of inertia of a golf ball by means of the difference between a period of oscillation of the golf ball when the golf ball is mounted in a fixture of the measuring instrument and a period of oscillation of the golf ball when the golf ball is not mounted therein.

The distance of the golf ball was tested using the UBL which is a launcher available from Automated Design Corporation. The test was performed under the conditions of an initial velocity of the ball of 59 m/s, the initial spin rate of 2000 rpm in a case in which the ball is launched so that the ball spins around the z-axis or 3000 rpm in a case in which the ball is launched so that the ball spins around the x-axis or y-axis, and the launch angle of 11 degrees. The results of the distance test in Table 1 show improved distances (unit: m) of Examples compared with the distance of Comparative Example 2. The UBL is an apparatus having two pairs of drums, top and bottom, the top and bottom pairs of drums having belts wound therearound, a ball being inserted therebetween, and launched under desired conditions.

As shown in Table 1, Example 1 resulted in extending the distance by 3.2 m compared with Comparative Example 2 in a case in which the ball was launched under a lower initial spin rate of 2000 rpm so that the ball spun around the z-axis, and in extending the distance longer by 2.3 m compared with Comparative Example 2 in a case in which the ball was launched under a higher initial spin rate of 3000 rpm so that the ball spun around the x-axis or y-axis, because the golf ball of Example 1 included a high specific gravity member having a specific gravity of 9.60, and thus, the moment of inertia with respect to the z-axis Iz was greater by 7 than the moment of inertia with respect to the x-axis or y-axis Ix or Iy, whereas the golf ball of Comparative Example 2 had a uniform moment of inertia with respect to the three axes.

Example 2 resulted in extending the distance by 2.3 m compared with Comparative Example 2 in a case in which the ball was launched under a lower initial spin rate of 2000 rpm so that the ball spun around the z-axis, and in extending the distance longer by 0.3 m compared with Comparative Example 2 in a case in which the ball was launched under a higher initial spin rate of 3000 rpm so that the ball spun around the x-axis or y-axis, because the golf ball of Example 2 included a high specific gravity member having a specific gravity of 4.97, which is lower than that of Example 1, and thus, the moment of inertia with respect to the z-axis Iz was greater by 3 than the moment of inertia with respect to the x-axis or y-axis Ix or Iy.

Example 3 resulted in extending the distance by 3.2 m compared with Comparative Example 2 in a case in which the ball was launched under a lower initial spin rate of 2000 rpm so that the ball spun around the z-axis, and in extending the distance by 2.3 m compared with Comparative Example 2 in a case in which the ball was launched under a higher initial spin rate of 3000 rpm so that the ball spun around the x-axis or y-axis, because the golf ball of Example 3 included the same high specific gravity member as Example 1 but included a core composed of plural layers which included a core layer having a low specific gravity, and thus, the difference of the moments of inertia increased to 8.

Comparative Example 1 resulted in extending the distance by 0.9 m compared with Comparative Example 2 in a case in which the ball was launched under a lower initial spin rate of 2000 rpm so that the ball spun around the z-axis, but in not extending the distance compared with Comparative Example 2 in a case in which the ball was launched under a higher initial spin rate of 3000 rpm so that the ball spun around the x-axis or y-axis, because the golf ball of Comparative Example 1 included a specific gravity member of 2.77, which is lower than that of Examples 1-3, and thus, the difference of the moments of inertia is only 1. 

1. A multiple-piece golf ball comprising: a core positioned at the center of the golf ball; an intermediate layer surrounding the core; a cover further surrounding the intermediate layer; and at least one high specific gravity member having a higher specific gravity than the core, the intermediate layer and the cover; wherein the at least one high specific gravity member is arranged such that a moment of inertia of the golf ball with respect to an x-axis Ix and a moment of inertia of the golf ball with respect to a y-axis Iy are substantially the same, a moment of inertia of the golf ball with respect to a z-axis Iz is greater than Ix and Iy, and the difference between Iz and Ix or Iy is at least approximately 3, the x-axis, y-axis, and z-axis intersecting orthogonally at the center of the golf ball.
 2. The golf ball according to claim 1, wherein the number of high specific gravity members is one.
 3. The golf ball according to claim 1, wherein the number of high specific gravity members is two or more.
 4. The golf ball according to claim 1, wherein the at least one high specific gravity member has a specific gravity of at least about
 2. 5. The golf ball according to claim 1, wherein the at least high specific gravity member comprises a metal or a metal compound. 